U.S Customs and Border Protection are making a change to current ESTA applications, meaning that if you affected, you will need to reapply for your ESTA before going on holiday.
"The ESTA website will be updated on August 5, 2019. All ESTA applications that are in a pending payment status at the time of the update will be archived. Applicants impacted will have to reapply after the update is complete. Citizens of participating Visa Waiver Program (VWP) countries traveling to the United States are strongly encouraged to apply for an ESTA at the time of booking their trip and no later than 72 hours prior to departure."
You can apply and check the status of your ESTA online with your passport number, date of birth and passport issue and expiry dates at https://esta.cbp.dhs.gov/esta/